1. Field of the Invention
The present invention relates to a power controller for a semiconductor laser device which is incorporated in a laser beam scanner used in, for example, a laser printer.
2. Description of the Related Art
In general, a semiconductor laser device is available as an integrated-circuit package or chip on the market, and includes a laser diode and an internal photodiode, which are integrally assembled in a small substrate. As is well known, the laser diode produces and emits two lasers from the opposite end faces thereof. Only one of the two lasers, i.e. a so-called front laser, is utilized as a scanning laser beam, and the other laser, i.e. a so-called back laser, is utilized to control a power of the front laser in a feedback manner.
Powers of the two lasers, emitted from the laser diode, are unstable because they are especially influenced by a variation in a temperature of the laser diode per se. In order to make the power of the front laser, and therefore, the power of the scanning laser beam, stable, the semiconductor laser device is associated with a feedback control circuit, a so-called auto-power control (APC) circuit, and an electric current power source for electrically energizing the laser diode is controlled by the APC circuit.
In particular, a power of the back laser is detected by the internal photodiode integrally assembled in the small substrate of the semiconductor laser device, and the internal photodiode produces an electric current in accordance with the detected power of the back laser. The produced electric current is outputted to the APC circuit, and is then converted into a voltage representing the detected power of the back laser. The APC circuit compares the converted voltage with a reference voltage, and controls an output current of the electric current power source for electrically energizing the laser diode such that a differential voltage between the converted voltage and the reference voltage becomes zero, thereby making the power of the scanning laser beam stable.
Nevertheless, the internal photodiode, integrally assembled in the small substrate of the semiconductor laser device, is inferior in ability of response. Therefore, for example, when the semiconductor laser device is incorporated in a laser beam scanner used in a laser printer, a printing speed of the laser printer cannot be increased greater than the ability of response of the internal photodiode.
It has been proposed that an external photodiode, such as a PIN-type photodiode, which is superior in ability of response to the internal photodiode, be associated with the semiconductor laser device, thereby further increasing the printing speed of the laser printer. In particular, the front laser, emitted from the laser diode, is optically shaped and converted into a laser beam, which is then split into two laser beams by, for example, a beam splitter. One of the split laser beams is utilized as a scanning laser beam, and a power of the other split laser beam is detected by the external photodiode to control an output current of the electric current power source for electrically energizing the laser diode in the same manner as mentioned above.
In addition to the superior ability of response, the APC control system using the external photodiode exhibits another merit that the power of the laser beam, detected by the external photodiode, directly represents the power of the scanning laser beam, whereby the output current of the electric current power source for the laser diode can be more properly controlled in comparison with the internal photodiode of the semiconductor laser device.
Nevertheless, the APC control system using the external photodiode involves a serious risk that the laser diode may be easily damaged when the output current of the external photodiode is accidently reduced. In particular, for example, during inspection and maintenance of the laser printer, from which a cover or housing is removed, the laser beam to be detected by the external photodiode may be accidently blocked off by a tool, such as a screw driver, foreign matter or the like. Of course, when the blockage of the laser beam to be detected takes place, the output current of the electric current power source for the laser diode is abruptly and excessively increased due to the feedback control of the APC control circuit, so that the laser diode is electrically energized with the excessively-increased current, resulting in damage to the laser diode.